Various portable electronic measuring instruments are currently available for which low power utilization is an important design consideration. One example of such a device is a displacement measuring instrument, such as a hand-held electronic caliper that can be used for making precise geometric measurements, such as that shown in U.S. Pat. Nos. 5,901,458, and 5,886,519, each of which is commonly assigned and hereby incorporated by reference in its entirety. The '519 patent discloses an inductive absolute position transducer for high accuracy applications, such as linear or rotary encoders, electronic calipers and the like. Such devices may utilize low power circuits, such as those shown in U.S. Pat. Nos. 6,859,762 and 6,747,500, each of which is commonly assigned and hereby incorporated by reference in its entirety. It is obvious that the less power such instruments use, the fewer batteries (or other power sources) they will require and the longer they will operate before the batteries (or other power sources) need to be replaced or replenished. However, reducing the power requirements of such devices is a complex task. Such devices are required to make highly accurate measurements, and the signal processing techniques that have been developed for such are required to both accomplish the desired accuracy and operate at low voltage and power levels, and be relatively insensitive to reasonable variations in supply voltage and operating temperature.
When such devices are operated at low power levels, one type of circuit that may be utilized to monitor the power to make sure it is above a minimum threshold is a power-on reset (POR) circuit, such as those disclosed in U.S. Pat. Nos. 7,161,396 and 7,015,744. As described in the '396 patent, most integrated circuit devices include a power-on reset circuit that asserts a reset signal when a supply voltage is detected and then de-asserts the reset signal when the supply voltage has reached an acceptable level that is sufficient for the device's normal operation. The power-on reset circuit can also be used to assert the reset signal when the supply voltage falls below an acceptable level. When asserted, the reset signal is typically used to reset the device's internal logic to a known state. When de-asserted, the reset signal is typically used to terminate the reset operation and allow the device to commence normal operation.
Many power-on reset circuits include both a voltage based circuit and a time-delay based circuit. The voltage based circuit is intended to reset the circuit when the supply voltage is too low, by generating a reliable reset signal when a slow rising power on is encountered. It also prevents the circuit from entering an undefined state if the voltage drops, by resetting it when the supply voltage goes below a minimum threshold. In contrast, the time-delay based circuit provides a reset pulse in the case of a fast rising power on. The two outputs from the voltage-based and time-delay based circuits are combined to provide an overall reset signal.
For certain applications, different implementations of the voltage-based and time-delay based circuits may be utilized. For example, in the case of discrete systems, a fixed threshold voltage implementation may be preferred for the voltage-based circuit, in that each integrated circuit of the discrete system will typically have a specified minimum operating voltage, and the reset signal can be set according to the specified minimum operating voltages. However, one drawback of this solution is that the specified minimum operating voltage may in some cases be inaccurate, in that the actual minimum operating voltage (i.e. when the supply voltage VDD is above the threshold for reliable circuit operation), will vary with process and temperature. In such cases, if the fixed threshold voltage for triggering the reset signal is set at an incorrect level, then power inefficiencies or circuit failure may result.
The present invention is directed to a customizable power-on reset circuit that determines the minimum power levels needed to reliably operate critical circuits of an integrated circuit, such as may be included in low voltage low power devices.